We have probably all seen these creatures somewhere. After all, the image of a pink undeveloped marsupial embryo living outside its mother’s womb is a staple of nature shows and biology films. The fact that these mammals do the majority of their developing outside of the womb is enough to set them apart, but new research out of Duke University shows that it is more than just where they develop that is different; their timeline of development is different (1).
When a marsupial embryo emerges from the birth canal it needs to get to its mother’s teat, which is its nutrient source while it finishes developing. To do this, the pink, rubbery-looking creature drags itself across its mother’s belly using a pair of surprisingly burly forelimbs. No other vertebrae embryo has such well-developed limbs at this stage of development! In fact in marsupials, limb development begins before that of almost every other structure; placing the limbs at a completely different place in the developmental timeline.
The Duke researchers found that some genes were activated much earlier in marsupials than in other animals. Furthermore, they showed that the forelimbs received cells from a larger part of the developing embryo than is seen with other vertebrates. Surprisingly, this early development wasn’t limited to the forelimbs, the genes for the hind limbs also turned on early. This pattern of gene expression establishes where the limbs will be, but the marsupial embryos devote a lot more resources (i.e., a higher number of early cells) to developing the necessary forelimbs.
When they emerge from the womb, marsupial embryos are blind, hairless and have an incomplete brain, and while they have hind limbs, these limbs are still small and rubbery. Few, if any, other animal embryos could survive outside the womb at this state of development. However, this process works for marsupials partly because of their unique developmental schedule, which ensures they emerge with forelimbs that have bones and developed muscles. Their developmental program has been wired so that it provides what they need to survive, and in the process it has given them a leg up on the other vertebrates (developmentally speaking).
- Keyte, A. L. and Smith, K. K. (2010) Developmental origins of precocial forelimbs in marsupial neonates. Development 137, 4283–4294.